In the development of new products within the field of lighting much effort is made to design light sources producing a full spectrum, i.e. a light output including all wavelengths of visible light. There is also a demand for light sources with a continuous spectrum showing black body emission at various correlated color temperatures (CCTs). Such full spectrum lighting is white light having a smooth intensity spectrum without sharp spikes or dips. This demand is based on the insight that while daylight is the best light, a continuous full spectrum artificial light is the second best. There are many claimed benefits of a continuous full spectrum, for examples that full spectrum lighting improves color perception, improves visual clarity, improves mood, improves productivity, improves mental awareness, increases retail sales, improves plant growth, improves results of light therapy in treating seasonal affective disorder (SAD) and sleep disorders, improves scholastic performance of students, improves vitamin D synthesis in the body, and reduces incidence of dental decay.
Various incandescent claimed full-spectrum lamps are commercially available such as a fluorescent T12 lamp. However, the spectra of these incandescent lamps are still showing spikes and/or dips. In addition, incandescent claimed full-spectrum lamps are also relatively energy-consuming.
LEDs emitting different colors (without phosphors) can be used to a obtain desirable CCT and CRI. However, the spectrum obtained with such direct emitters is very peaked with large dips, see FIG. 13. Using direct LEDs no full spectrum lighting can be made. Another drawback of using different direct LEDs is that each LED needs a different driving current. Furthermore, due to different temperature dependencies of different LEDs would require the current for at least some of the LEDs to be adjusted as a function of the temperature).
Using phosphor-converted light emitting diodes (LEDs) it is also difficult to obtain full spectrum lighting without spikes and/or dips in the spectrum. In phosphor-converted LEDs blue light is partially converted to yellow/orange/red light in order to obtain white light. However, the spectrum with such phosphor converted LEDs always show a peaked spectrum with dips. FIG. 12 shows a spectrum of blue LED which is partially converted to yellow and red light by a yellow and red phosphor, respectively, in order to obtain white light having CCT of 300 0K and a color rendering index (CRI) of 90. It is difficult to fill in the gaps of this spectrum using conventional organic and inorganic phosphors, which are broad band emitters, to obtain continuous full back body emission.
US 2005/0135079 suggests an LED device for a flash module which produces white light with a higher CRI than prior flash modules. The device comprises a light source producing primary light and a wavelength converting overlay including a plurality of quantum dots dispersed in a matrix material. The quantum dots may be selected to have different secondary emission wavelengths to produce a broad emission from the light emitting device. In some embodiments, quantum dots are combined with conventional phosphor material. However, a drawback of the device described in this document is that re-absorption of the secondary light may lead to reduced efficiency and it becomes difficult to make a fine tuning of a desired spectrum.
Hence, there remains a need in the art for improved full-spectrum light sources.